There are a variety of X-ray sensitive materials which may be used in X-ray imaging devices such as Vidicon tubes. Typical among them is thallium bromide, Tl Br, which is characterized by a high dielectric constant (.apprxeq.30) and excellent X-ray sensitivity. (Others are thallium iodide, thallium bromo-iodide, lead bromide and lead iodide.) However, these materials, exemplified by thallium bromide, have been shown to exhibit high electrical conductivity at ambient temperature (say, 27.degree. C.) which produces an acceptable dark current. The ionic conductivity of thallium bromide is highly temperature dependent and the ionic conductivity can be reduced to acceptable levels by active cooling.